Samlex America PST-200S-24E - Manual (Page 20)

20 | SAMLEX AMERICA INC.
Table 7.3 (page 19) will show that a 100 Ah capacity battery will deliver 100% (i.e. full
100 Ah) capacity if it is slowly discharged over 20 hours at the rate of 5 Amperes (50W
output for a 12V inverter and 100W output for a 24V inverter). However, if it is dis-
charged at a rate of 50 Amperes (500W output for a 12V inverter and 1000W output for
a 24V inverter) then theoretically, it should provide 100 AH ÷ 50 = 2 hours. However, the
Table above shows that for 2 hours discharge rate, the capacity is reduced to 50% i.e. 50
Ah. Therefore, at 50 Ampere discharge rate (500W output for a 12V inverter and 1000W
output for a 24V inverter) the battery will actually last for 50 Ah ÷ 50 Amperes = 1 Hour.
State of charge (Soc) of a Battery – Based on “Standing Voltage”
The “Standing Voltage” of a battery under open circuit conditions (no load connected
to it) can approximately indicate the State of Charge (SoC) of the battery. The “Standing
Voltage” is measured after disconnecting any charging device(s) and the battery load(s)
and letting the battery “stand” idle for 3 to 8 hours before the voltage measurement
is taken. Table 7.4 below shows the State of Charge versus Standing Voltage for a 12V
battery system at around 80°f (26.7ºC). for 24-volt systems, multiply by 2; for 48-volt
systems, multiply by 4.
tABLe 7.4: StAte oF cHARGe VeRSUS StAnDinG VoLtAGe – 12V BAtteRY
Percentage of
Full charge
Standing Voltage of 6 cell,
12V nominal Battery
Standing Voltage
of individual cells
100% 12.63V 2.105V
90% 12.6V 2.10V
80% 12.5V 2.08V
70% 12.3V 2.05V
60% 12.2V 2.03V
50% 12.1V 2.02V
40% 12.0V 2.00V
30% 11.8V 1.97V
20% 11.7V 1.95V
10% 11.6V 1.93V
0% = / < 11.6V = / < 1.93V
Check the individual cell voltages / specific gravity. If the inter cell voltage difference
is more than a 0.2 V, or the specific gravity difference is 0.015 or more, the cells will
require equalization. Please note that only the non-sealed / vented / flooded / wet cell
batteries are equalized. Do not equalize sealed / VRLA type of AGM or Gel Cell Batteries.
State of Discharge of a loaded battery – Low Battery /
Dc input Voltage Alarm and Shutdown in inverters
Most inverter and UPS hardware estimate the State of Discharge of the loaded battery
by measuring the voltage at the inverter’s / UPS’s DC input terminals (considering that
seCtIOn 7 | General Information on Batteries for
Powering Inverters